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Confirmed cases of Zika virus in Singapore have risen to 82, as the United States joined a growing list of countries warning pregnant women or those trying to get pregnant to avoid travel to the city-state.

The mosquito-borne Zika virus, which has caused explosive outbreaks in the Americas and the Caribbean since late last year, poses a particular risk to pregnant women because it can cause microcephaly, a severe birth defect in which babies are born with abnormally small heads and underdeveloped brains.

The United States warned pregnant women not to travel to Singapore, joining Australia, Taiwan and South Korea.

The warnings followed news that Zika transmission appears to be occurring outside of the original cluster, with at least five of 26 new cases confirmed detected in the Aljunied area in the southeast of Singapore, the Health Ministry and National Environment Agency (NEA) said.

Singapore has advised pregnant women to take a free Zika test if they showed any symptoms or if their partners tested positive.

"This is regardless of whether they have been to Zika-affected areas," health officials said in a statement. The outbreak and advisories come as the tourism industry in one of the world's busiest travel hubs already faces weak global economic growth.

Singapore's Tourism Board said it was premature to consider any impact on the sector, adding it remained a "safe travel destination".

More than 55 million people pass through Singapore's Changi Airport every year. In the first half of this year, tourism arrivals topped 8 million, around 1 million more than a year earlier.

Singapore reported its first case of locally-transmitted Zika at the weekend, and the number of confirmed infections has risen steadily since then. At least three dozen patients have made a full recovery.

Neighbouring Malaysia and Indonesia have stepped up protective measures, introducing thermal scanners at airports and border checkpoints with the island state.

Such measures, however, will only identify symptomatic cases of Zika, but only one in five people have symptoms of the virus, which include fever, rash, joint pain and pink eye.

Singapore residents responded to government calls to be vigilant and to take precautions against mosquito bites. Online retailer Lazada Singapore said sales of insect repellents jumped fivefold in the past three days.

FairPrice supermarkets and Watsons pharmacies said their sales of such products had doubled. Most of the early infections were among foreign workers, hundreds of thousands of whom, mainly from the Asian sub-continent, work on Singapore's construction sites and in the marine sector.